Math From MS in Math to software engineering, experience with webapps

First, I'll describe my situation. Also, I apologize if this is the wrong place to ask my question. If it is, please let me know, and (if possible) suggest a more appropriate venue for my question.

I graduated with a BS in Math a few years ago. I worked for 4 years for a web app company (mostly PHP4, Postgres, PrototypeJS, HTML4) where I wrote our in-house DAL and events framework, became the SVN admin, wrote & implemented our REST API, and was heavily involved with setting up our load balancers and maintaining the server farm, and of course wrote various pages for the webapp.

Then I went back to school to get my PhD in Math, but I've decided to settle with an MS and leave. Most of my courses are purely math theory, though one is numerical analysis and scientific computing in which we use MATLAB to some small degree.

Now I would like to break into software engineering, probably something in big data or scientific computing.

This brings me to my questions:

1) How valuable will my previous webapp experience help me when interviewing for big data or scientific computing jobs?

2) If I want to start work in June or July, what can I do in the meantime to increase my chances of getting the sort of job I described?

3) Based on your experiences, would my earning potential be higher if I stayed in webapps, or moved to big data/scientific computing? Or would the earning potential be about the same?

1) How valuable will my previous webapp experience help me when interviewing for big data or scientific computing jobs?

For the company I work for, it would be useful because it demonstrates you can program. We primarily hire statisticians that can program (as opposed to programmers that know some statistics), for the 'big/data/ type positions, so we would expect you to come in able to answer some questions about common analysis methods (maybe glms, and maybe some non-linear model like a neural net or random forest).

2) If I want to start work in June or July, what can I do in the meantime to increase my chances of getting the sort of job I described?

Get a data set (plenty of common data sets are available- look at sites like Kaggle or some of the built in data sets in R), and go through some analysis. Pre-process the data to select good variables, test some models. Be prepared to answer questions about your choices of model, etc.

3) Based on your experiences, would my earning potential be higher if I stayed in webapps, or moved to big data/scientific computing? Or would the earning potential be about the same?

I have no idea what a web developer makes. I've been able to get to six figures fairly quickly in a big-data position.